1. Field of Invention
The present invention relates to a design and fabrication method of a laser transmitting apparatus. More particularly, the present invention relates to a design and fabrication method of a laser transmitting apparatus for utilizing a continuous mode laser driver circuit for a burst mode transmission application where the high level and the low level of the laser power eye diagram are stably maintained so as to decrease the bit error rate in transmission process.
2. Description of Related Art
In the past, signals of optical fiber communication were usually transmitted in continuous mode. In continuous mode, since no signal can be interrupted during the transmission process, the optical fiber communication structure can only be that in which one receiver connects to one transmitter. This structure is also called a P2P (point to point) structure.
However, if FTTX (fiber to the X, where X represents a home, building, etc.) is used, the signals do not need to be transmitted all the time, and thus it is a waste of communication bandwidth to use a P2P communication apparatus.
Thus, optical fiber communication system designers have developed a FSAN (full service access network) structure for FTTX applications. In FSAN, an important concept is P2MP (point to multi points). When a user wants to transmit some information, the user is allowed to transmit the information immediately. After the completion of transmitting the information, the user is allowed to turn the laser power off to avoid signal conflicts. Such transmission is called burst mode, which is different from the continuous mode mentioned above.
For an optical fiber communication system used for burst mode applications, a major component is the laser driver chip. For burst mode laser driving, the laser driver circuit must be able to turn on in a very short time and drive a communication laser diode to transmit a signal and then be able to turn off the laser diode completely and rapidly when there is no signal input.
In the FSAN standard, there is no clear definition for the signal present time and stop time in burst mode. Therefore, system integration companies have developed many different solutions. Some of the solutions need a strict timing control. Thus, the laser driver chip is difficult to assemble and the production cost is also increased.
In burst mode optical fiber communication systems, when the NZR (non return to zero) data rate is lower than 1.5 Gbps, direct modulation of the laser driver can be used. When the NRZ data rate is higher than 1.5 Gbps, an external modulator is needed. This invention concerns burst mode optical fiber communication in which the NRZ data rate is lower than 1.5 G.
In direct modulation, a bias current and a modulation current are needed for a laser diode. Usually, a laser driver chip is responsible for providing the bias current and the modulation current. The laser driver circuit receives the input voltage signal and then converts the input voltage signal into a current.
For continuous mode, many types of integrated circuit are available on the market, but for burst mode, only a few specially designed integrated circuits are available. The specially designed integrated circuits are, for example, chips having DAPC (digital automatic power control) functionality or chips having external bias capacitors.
The external bias capacitor is used to control the stability of the laser light. The specially designed integrated circuits mentioned above have the disadvantages of high price and long bias current setting time.
Therefore, a laser transmitting apparatus that does not need digital automatic power control or an external bias capacitor is in great need, one that only requires the automatically adjusted modulation current and bias current in response to the environmental temperature, so that the high level and the low level of the laser power eye diagram of the laser power are stably maintained.